Laminated veneer lumber (LVL) is composed of layers or plies of wood veneer bonded one veneer layer to the next by an adhesive and with the grain direction of all veneer plies parallel to one another. Normally, LVL has higher stress design values than solid wood for structural applications.
While veneers can be made by either a slicing or a rotary peeling operation, nearly 95% of all veneers produced in the world today are rotary peeled because of higher production efficiency. All known commercial LVL's for structural applications are produced from rotary peeled veneers.
Laminated veneer lumber made from softwoods is well documented in the prior art. In 1944, R. F. Luxford showed that Sitka Spruce laminated veneer products had strength properties equal to solid wood material. Reference may be had to an article entitled "Strength of Glued Laminated Sitka Spruce Made Up of Rotary Cut Veneers", U.S. department Agri Forest Service FPL 1512 Forest Prod Lab, Madison Wis. MICRO=LAM* lumber, a laminated veneer lumber was developed by Trus Joist Corporation and introduced to the construction industry in 1971 in the U.S.A. LVL has been made from rotary-peeled Douglas-fir veneers and more recently from Southern yellow pine veneers. A laminated veneer lumber (LVL) made from spruce veneer and known in the trade as Master Plank* was sold by Gang-Nail Systems Inc. in the U.S. and more recently are selling LVL known as Gang-Lam made from Southern Yellow Pine veneers. A Spruce laminated veneer lumber (LVL) has been produced in Finland since 1981 and known in the trade under the name Kertopuu. All of these LVL's are made from softwoods, i.e., Spruce, Pine or Douglas Fir.
Patents relating to LVL that have issued, are by way of example, U.S. Pat. No. 4,608,106 issued Aug. 26, 1986 to P. K. Lahtinen U.S. Pat. No. 3,908,725 issued Sept. 30, 1975 to P. Koch and U.S. Pat. No. 3,580,760 issued May 25, 1971 to P. Koch.
None of the prior art teaches using veneers of different wood species or how the veneer layers should be laid up relative to one another to make LVL from mixed wood species. Furthermore, the known LVL is used exclusively for structural purposes.
In using rotary peeled veneer and/or different species there are potential problems that must be considered. For example, warping and cracking can occur due to differential shrinkage and expansion between different species
With respect to rotary peeling a log, the veneer is unwound in a long continuous ribbon at a pre-set thickness. As a result, the direction perpendicular to the grain direction and thickness direction of a veneer is the tangential direction to the annual rings, the thickness direction of a veneer is the radial direction to the annual rings and the direction along the wood grain direction is the longitudinal direction of a veneer. For simplicity these are defined as tangential (or width) direction, radial (or thickness) direction and longitudinal (or length) direction, respectively. It is a fact that the dimensional change of a veneer is the greatest in the tangential direction. Therefore, the dimensional changes in the width (tangential) direction of LVL could be noticeable and affect the integrity of the structural system when the LVL is subjected to severe moisture changes.
Lathe checks are inevitable in every laid out, flat, rotary-peeled veneer. The fractures that develop in the veneer are due to the severe veneer bending required following the cutting action of the knife during rotary peeling to provide a flat sheet. This bending of the veneer during peeling is against the natural curve of the veneer.
In view of the above facts, LVL can have large dimensional changes in its width direction and cracks from the propagation of lathe checks in the veneer when it is subjected to even minor moisture changes. To develop sufficient adhesion between veneers, a veneer densification inevitably occurs and some internal stresses develop within the LVL and thus the thickness swelling of LVL becomes a concern when it is exposed to water or high humidity environment. Consequently, it is desirable to improve the dimensional stability and durability of LVL.
As indicated hereinbefore, LVL's are either produced from a single species (e.g. Douglas Fir or Spruce) or from the same group of species (e.g. Southern Yellow Pines, e.g., long leaf pine, short leaf pine). Also, currently all LVL is for structural applications and is exclusively made from softwoods. However, many hardwoods are superior to softwoods in terms of strength, toughness, hardness and abrasion resistance, and thus superior LVL could be produced from many hardwoods. Unfortunately, it is difficult to economically obtain large quantities of single species hardwood veneers for a LVL plant.
Mixed species forest stands are common in many regions of the world. Normally, only one or two species are desirable for sawmills or plywood mills and a lot of trees are left as residues after harvesting. It is costly to clear the forest sites after the primary species of trees are harvested. It is also a great waste from the viewpoint of resource utilization. High quality hardwood veneers are highly desirable in furniture and cabinet industries but lower grades of hardwood veneers have very few market niches. This provides a great pressure on the hardwood veneer producers. These under-utilized grades of hardwood veneers can be made (with other species of veneers) into LVL for structural and nonstructural applications in accordance with the present invention.